Today's storage systems are used in computing environments for generating and storing large amounts of critical data. The storage capacity of these systems has increased over time such that some storage systems support many—sometimes hundreds—of disk drives. Because disk drives are mechanical devices, they have higher peak power requirements than the other electronic devices in the system. In today's market it is important to design the power subsystem portion of a storage system to support the maximum and peak power requirements of the system while minimizing the cost of the system.
A typical disk drive consists of circuit board logic and a Hard Disk Assembly (HDA). The HDA portion of the disk drive includes the spindles platters, head arm and motor that make up the mechanical portion of the disk drive. When power is applied to the disk drive, the circuit board logic powers up and the HDA spins up. During spin up, the HDA requires a higher current than when it is in steady state—i.e. already spun up. This extra current is typically more than two times the steady state current. Therefore, if a storage system at power up attempts to spin up all the drives in the system at the same time, the system is required to support a peak power level that is much greater than the maximum power required to operate at steady state. The more disk drives the system supports, the greater the peak power requirement. It is too expensive to provide a power subsystem that can support enough peak power to power up all disk drives at once, especially when the excess power is not otherwise needed.
Some types of disk drives offer a separate HDA power input but no built in control over the timing of the application of power to the HDA. Some other types offer limited control. For example, Fibre Channel disk drives compliant with the SFF-8045 rev. 4.7 standard allow the timing of HDA spin up to be controlled via two signal pins, Start_1 and Start_2, that allow the HDA to spin up based on three different conditions. Depending on the state of the Start—1 and Start—2 signals, the disk drive HDA will start drawing current either 1) immediately; 2) after it receives its first SCSI command, or 3) based on its arbitrated loop physical address (ALPA). This provides limited means to stagger the timing of spin up amongst all the drives such that system peak power requirements can be minimized.
However, it is difficult for system software to use SCSI commands to control drive spin up timing, because insert and power control signals from the drives are asserted much faster than software can respond. The ALPA address method is also disadvantageous in some circumstances. Consider a storage system capable of supporting 48 disk drives. If a user plugs a single disk drive into the 48th slot in a system wherein the other drives are all at steady state, there is no reason why the drive should not spin up immediately. But because its spin up timing depends on its ALPA address, it will nonetheless take several minutes to spin up.
What is needed is a mechanism for controlling the application of power to the HDA portion of disk drives in a storage system. The mechanism must be more efficient than current methods and applicable regardless of the type of disk drives employed.